1. Field of the Invention
The present invention relates to a single crystal pulling apparatus for manufacturing a dislocation-free single crystal of silicon by Czochralski method.
2. Description of the Prior Art
In a single crystal manufacturing apparatus based on the pulling Czochralski (CZ) method, fresh argon (Ar) gas is supplied into a highly pressure-proof airtight chamber where pressure is reduced to about 10 torr in advance and polycrystal in a quartz crucible arranged in the lower portion of the chamber is melted by heating. Then, a seed crystal is immersed into surface of the melt from above, and by rotating and moving up and down the seed crystal and the quartz crucible, the seed crystal is pulled up. As a result, a single crystal (the so-called ingot) is grown, which comprises an upper cone portion with its upper end protruding, a cylindrical body portion, and a lower cone portion with its lower end protruding, all under the seed crystal.
As a method to grow as described above, Dash method is known. According to this method, in order to eliminate dislocation (i.e. in order to turn to dislocation-free), which occurs in the seed crystal due to thermal shock when the seed crystal is immersed to the surface of the melt, pulling rate is relatively increased after immersing the seed crystal to the surface of the melt so that a neck portion having smaller diameter than the seed crystal, e.g. 3 to 4 mm, is formed, and pulling of the upper cone portion is started.
Further, a single crystal having large diameter and heavy weight (150 to 200 kg or more) cannot be pulled up via the neck portion with small diameter, and a method has been proposed, for example, in JP-B-5-65477. According to this method, a neck portion with small diameter is formed by Dash method, and the pulling rate is then relatively slowed down and a portion with larger diameter is formed. Then, the pulling rate is relatively increased, and a portion with small diameter is formed. Thus, a "spherical constricted portion" is formed, and by gripping this constricted portion with a gripper, the single crystal having large diameter and heavy weight is pulled up. Also, a conventional type apparatus for gripping the constricted portion is proposed, for example, in JPB-7-103000 and JP-B-7-515.
As other conventional examples, a method for directly gripping a body portion with a "constricted portion" as described above has been proposed, for example, in JP-A-5-270974 or JPA-7-172981. Also, a method to form a "ring-like constricted portion" having a larger diameter than the body portion between the upper cone portion and the body portion instead of the above "spherical constricted portion" and to grip this "ring-like constricted portion" has been proposed in JP-A-63-252991 and JP-A-5-270975.
However, it is very delicate matter to determine the timing to grip the above "constricted portion" by a gripping member during pulling of single crystal. If the timing to grip is deviated, the single crystal under pulling operation may be turned to polycrystal. Also, a plurality of single crystal manufacturing apparatuses are usually aligned in the manufacturing factory, and in case a small number of workers are in charge of gripping operation, it may lead not only to poor working efficiency but also to a possibility to cause deviation in the timing to grip.
The gripper for gripping the "constricted portion" during pulling operation of single crystal is used repeatedly, and when the gripper is damaged due to fatigue, single crystal may be dropped down. Because the gripper is generally driven by a motor, gripping power is decreased during power suspension, and single crystal may be dropped down. In the method to grip the "constricted portion" as described above, if the "constricted portion" is damaged, single crystal may be dropped down. If single crystal is dropped down, dislocation may occur, and the single crystal is not qualified as a product any more. Also, when quartz crucible is damaged, high temperature melt may react in the worse case with the cooling water inside the crucible shaft, which rotates and moves up and down the quartz crucible. As a result, vapor explosion may occur.